This invention relates to the delivery of compounds through epithelial cell layers using impulse transients, i.e., stress waves.
Various methods have been employed for facilitating the delivery of pharmaceutical agents through the skin. One layer of the skin is the stratum corneum, which forms the outermost layer of the epidermis and is thought to act as the skin""s primary barrier to molecular transport. It has a thickness of 10 to 15 xcexcm and is composed of layers of corneocytes, with the layers varying in thickness from 10 to 50 cells. Corneocytes are keratin-filled cells that lack nuclei and cytoplasmic organelles. Intercellular regions of the stratum corneum are composed mostly of neutral lipids and comprise 5 to 21% of the stratum corneum volume.
One method of delivering drugs through the skin is iontophoresis, in which electric current applied to the surface of the skin increases the penetration of charged drugs (Singh et al., Med. Re. Rev., 13:569, 1993). However, the efficiency of drug delivery using this method depends on the ionization state of the drug. In addition, because iontophoresis uses high current densities, it can burn the skin (Singh et al., supra).
In another method, phonophoresis, a drug is delivered through intact skin using ultrasound (Skauen et al., Intern. J. Pharm., 20:235, 1984; Mitragotri et al., J. Pharmaceut. Sci., 84:697, 1995). However, the tensile component of ultrasound waves (negative pressure), which is always present in ultrasound waves, can cause tissue injury (Ter Haar, Biological Effects of Ultrasound in Clinical Applications, In Ultrasound: Its Chemical, Physical, and Biological effects, Suslick, ed., VCH Publishers, pp. 305-20; 1988). In addition, the method requires long exposure to deliver a therapeutic dose of the drug.
The invention is based on the discovery that high pressure impulse transients, e.g., stress waves (e.g., laser stress waves (LSW) when generated by a laser), with specific rise times and peak stresses (or pressures), can safely and efficiently effect the transport of compounds, such as pharmaceutical agents, through layers of epithelial tissues, such as the stratum corneum and mucosal membranes. The new methods can be used to deliver compounds of a wide range of sizes regardless of their net charge. In addition, impulse transients used in the methods avoid tissue injury.
The compounds that can be transported through epithelial tissue layers by the new methods include pharmaceutical compounds such as photosensitizers, anesthetic agents, polypeptides, nucleic acids, and antineoplastic agents such as cisplatin, and mixtures of compounds.
In general, the invention features a method of delivering a compound, e.g., an anesthetic, such as lidocaine, a hormone, such as insulin, an anti-neoplastic agent, or a nucleic acid, through an epithelial tissue layer by (a) mixing the compound with a coupling medium to form a compound-coupling medium mixture; (b) contacting a surface of the epithelial tissue layer with the compound-coupling medium mixture; and (c) propagating one or more impulse transients through the compound-coupling medium mixture to contact and enter the epithelial tissue layer, whereby the compound passes through the epithelial tissue layer.
Each impulse transient can be a broad-band compressive wave having a rise time of at least 1 ns and a peak pressure of at least 300 bar less than that which will damage tissues, e.g., about 2000 bar. In certain embodiments, the impulse transient can have a duration of about 100 ns to 1 microsecond. The impulse transient can be generated by exposing a target material to a pulsed laser beam. The method can be enhanced by adding a step of applying hydrostatic pressure.
In certain embodiments, a transparent material can be bonded to a surface of the target material to enable confined ablation. In other embodiments, the target material can be a metallic foil, e.g., of aluminum or copper, or a plastic sheet, e.g., of a polymer like polystyrene, and the impulse transient is generated by a laser-induced plasma formed by ablation of the target material. In another embodiment, the target material can be an absorbing material, and the impulse transient is generated by laser-induced rapid heating of the absorbing material.
In another aspect, the invention features an apparatus for delivering a compound through an epithelial tissue layer. The apparatus includes a reservoir for containing a coupling medium suitable for mixing with the compound, wherein the reservoir is arranged to enable the coupling medium to directly contact a surface of the epithelial tissue layer; and an energy source, e.g., a laser or lithotripter, arranged and controlled to propagate an impulse transient within the reservoir when filled with the coupling medium.
In another embodiment, the apparatus further includes a target material, e.g., a metal foil or plastic sheet, arranged between the laser and the reservoir, and the reservoir is configured to enable the target material to directly contact the coupling material in the reservoir. The apparatus can further include a transparent material bonded to a surface of the target material and interposed between the surface and the laser, and arranged to confine pressure forces resulting from ablation of the target material within the reservoir. The invention also features a system for delivering a compound through an epithelial cell layer in an animal. This system includes the apparatus and a coupling medium suitable for mixing with the compound.
The laser pulse can have a duration of about 10 to 70 nanoseconds (ns), or in certain embodiments, a duration of about 20 to 40 ns. About 1 to 10 laser pulses, and consequently 1 to 10 impulse transients, are applied to an epithelial cell layer during any one exposure period. In certain embodiments, about 1 to 3 laser pulses are applied.
The impulse transients can have a rise time of about 1 to 200 ns. Typically, the impulse transients can have a rise time of about 5 to 15 ns.
The impulse transients can have a peak stress or pressure of about 300 to 2000 bars, depending on the nature of the epithelial cell layer. In particular embodiments, the impulse transients can have a peak stress or pressure of about 500 to 1500 bars, e.g., about 550 to 650 bars.
The impulse transients can have a duration of about 100 ns to 1.1 microseconds (xcexcs). In specific embodiments, the laser pulse can have a duration of about 150 to about 750 ns, or about 200 to about 300 ns.
An impulse transient is a broad-band, compressive wave having a peak pressure of up to about 2000 bar, and a fast, but not discontinuous, rise time (on the order of 200 ns or less). Accordingly, impulse transients are not shock waves, which are characterized by a discontinuous rise time. Further, an impulse transient is preferably a unipolar compressive wave, but in addition to the major compressive component, can include a minor tensile component that is less than 5 to 10% of the compressive peak pressure.
A coupling medium is a non-linear liquid or gel medium in which the impulse transients are generated and propagated. The coupling medium enables a direct contact of the impulse transients to the surface of the epithelial cell layer and minimizes acoustic reflections.
The coupling medium may optionally contain a surfactant to enhance delivery of the compound across the epithelial tissue, e.g., by increasing the time required for the epithelial tissue to become impermeable following generation of an impulse transient. The surfactant can be a detergent and thus can include, e.g., sodium lauryl sulfate, cetyl trimethyl ammonium bromide, and lauryl dimethyl amine oxide.
The invention has many advantages. In particular, the specific rise time and magnitude of the impulse transients used in the new methods induce a temporary permeability in epithelial tissue layers. This increases the diffusion of compounds through these layers for a short period of time, and allows effective delivery of the compounds such as drugs without causing destruction or killing of cells. Thus, the method can be used to deliver drugs to desired locations underlying epithelial cell layers. For example, impulse transients can be used to deliver chemotherapeutic agents to the site of a skin cancer lesion. In this manner, a host of maladies can be treated.
Moreover, drugs that have been previously dismissed because they could not be transported through epithelial tissue layers, e.g., the stratum corneum layer, can be delivered using the new methods. Similarly, the new methods can also be used to deliver drugs whose toxicity or high cost precludes or discourages systemic administration.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present document, including definitions, will control. Unless otherwise indicated, materials, methods, and examples described herein are illustrative only and not intended to be limiting.
Various features and advantages of the invention will be apparent from the following detailed description and from the claims.